Direct-injection, spark-ignition engines typically operate with injection pressures up to 150 bar. To obtain such a high pressure, it is common to employ two pumps in series: a low pressure, electrically-operated pump bringing the pressure to 3 to 7 bar feeding a high pressure pump. The high pressure pump can be electrically, hydraulically, or mechanically operated. Mechanical pumps tend to predominate as they are more efficient than electric pumps and less complex than hydraulic pumps. To provide variable control of a mechanical pump, it is known to install a solenoid valve in the pump which can be opened and closed during the pump plunger's travel. When the solenoid valve is open, the pump plunger merely pumps the fuel back to the inlet side of the pump. When the solenoid valve is closed, the pump's plunger causes the fuel to be pumped into the outlet side at high pressure. By controlling the solenoid valve based on the plunger position, the amount of fuel and pressure of the fuel delivered can be tailored to the demand of the engine.
The solenoid valve makes a clicking sound during each actuation. During normal driving, the clicking is not noticeable in relation to road, wind, and engine noise. However, at idle, when the engine is at its quietest and there is no road and limited wind noise, the clicking noise is noticeable and can be a source of customer concern or dissatisfaction. Thus, it is desirable to overcome that noise.
The inventors of U.S. Pat. No. 7,198,033 show one such solution. They teach closing the solenoid valve when the engine speed is low. By keeping the solenoid in the closed position, the stroke of the plunger in the high pressure pump is allowed to displace fuel into the high pressure line for the full extent of its stroke. This is in contrast to other operating modes in the solenoid is opened for at least a portion of the plunger stroke, thereby discontinuing the pump plunger's pumping ability while the solenoid if open. Because the engine demands a lower amount of fuel than the pump supplies at an idle condition, the inventors of '033 provide a pressure relief valve and a return line downstream of the engine. If such a return line were not provided, the pressure in the line would continue to rise. Eventually, the pump, fitting, or other fuel system component would fail, if such a return line were not provided.
The inventors of the present invention recognize that it is desirable to avoid having to provide such a fuel return line and pressure relief valve for the engine. Furthermore, the inventors recognize that the approach taken in '033 leads to excessive fuel heating. That is, by holding the solenoid valve in the closed position, the pump pressurizes and delivers a full stroke's worth of fuel. The amount not consumed by the engine is returned to the fuel tank at low pressure, which fuel is heated when the pressure is dropped and due to the fuel being in close proximity to the engine while being circulated.